Bacillary dysentery and specifically shigellosis is a global human health problem. It has been over 100 years since the discovery of Shiga's bacillus, yet shigellosis remains endemic in most areas of the world including industrialized nations. An estimated 200 million people worldwide suffer from shigellosis, with more than 650,000 associated deaths annually (27). A recent CDC estimate indicates the occurrence of over 440,000 annual shigellosis cases in the United States alone (32), approximately 80% of which are caused by Shigella sonnei. All virulent S. sonnei strains comprise a single serotype determined by form I O-polysaccharide (O-Ps). This O-Ps is composed of a disaccharide repeating unit containing two unusual amino sugars, 2-amino-2-deoxy-L-altruronic acid (L-AltNAcA) and 2-acetamido-4-amino-2,4,6-trideoxy-D-galactose (4-n-D-FucNAc) (25). The genes encoding the enzymes that produce this O-Ps are novelly located on the 180 kb virulence plasmid in S. sonnei (26), which also harbors the invasion genes (36). Virulent form I colonies are typically unstable and upon replating convert to rough colonies, termed form II, due primarily to spontaneous loss of the large virulence plasmid and the ensuing loss of form I O-antigen. Substantially identical genes that encode the same antigen producing enzymes are located on the bacterial chromosome in Plesiomonas shigelloides (termed the O17 gene cluster).
Immunity to Shigellae, acquired either by natural infection or volunteer challenge, is mediated largely by immune responses directed against the serotype specific O-Ps (9, 10). This insight has led to the development of a variety of candidate vaccines containing Shigella O-Ps for oral or parenteral administration including recombinant heterologous, live, bacterial carrier strains (3, 12, 18). Parenteral vaccines in the past have not been effective in protecting against bacillary dysentery because shigellosis is an infection limited to the superficial layer of the colonic mucosa. It is, therefore, not surprising that attempts to immunize man or other primates with killed whole cell Shigella vaccines, administered by the parenteral route, have not been successful.
In early recombinant vaccine efforts, the virulence plasmid of S. sonnei was transferred as part of a larger plasmid cointegrate to the attenuated vector Salmonella enterica serovar Typhi strain Ty21a (i.e. S. Typhi Ty21a) (12). The resulting hybrid vaccine strain, 5076-1C, expressed S. sonnei O antigen as a lipid-linked surface O-Ps as well as S. Typhi 9,12 LPS (37). Although not core-linked, this form I O-Ps was immunogenic (12) and oral immunization of volunteers with 5076-1C elicited protection against virulent S. sonnei oral challenge (3, 21, 40). However, the protection observed in volunteers was variable, presumably due to loss of the form I gene region from the large cointegrate plasmid in 5076-1C (17). Thus, further molecular studies are needed to stabilize the S. sonnei form I gene region in vaccine vector constructs. In spite of an increased molecular understanding of Shigella pathogenesis, there are still no licensed vaccines for protection against shigellosis in the United States.
Although the form I O-Ps-encoding locus has been studied in some detail previously (6, 24, 38, 42, 45) the biosynthetic pathway and minimal gene region for stable expression of O-antigen have not been unambiguously defined. We show through deletion and sequence analyses and LPS expression studies that the S. sonnei form I biosynthetic gene region comprises a 12.3 kb operon. A detailed biosynthetic pathway, based on DNA sequence analysis of this region and the known structure of form I O-Ps, is proposed. In addition, stable expression of form I O-Ps was observed from a low copy plasmid and was associated with the removal of an adjacent IS91 resulting in small, genetically stable form I gene region constructs. We report the development and animal testing of a live attenuated S. Typhi vaccine vector stably expressing enzymes that produce form I O-Ps for protection against S. sonnei disease.
To develop a more stable living attenuated oral Shigella strain vaccine, the gene region encoding the enzymes that produce form I antigen was isolated from a large non-conjugative plasmid and analyzed to determine the essential genes required for biosynthesis of Shigella sonnei form I O-polysaccharide. Nucleic acids totaling 18 kb, were characterized genetically and used to define a minimal region encoding all of the proteins required to produce the form I antigen for development of live vaccine vector strains. Constructs comprising a 12.2 kb region encoding a consensus promoter and ten contiguous ORF's, and additional flanking DNA were generated which contained all of the information required to produce the Shigella form I O-Ps antigen. Significantly, attenuated Salmonella enterica serovar Typhi live vector vaccine candidate strains, containing minimal-sized form I O-Ps operon constructs, elicited immune protection in mice against virulent S. sonnei challenge.